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# If 4x(5n) = t and x and n are integers, what is the value of t ?

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Intern
Joined: 11 Jan 2015
Posts: 34
If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2016, 05:33
2
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Difficulty:

65% (hard)

Question Stats:

61% (01:28) correct 39% (01:18) wrong based on 72 sessions

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If 4x(5n) = t and x and n are integers, what is the value of t ?

(1) t is a perfect square less than 250.
(2) xn = 5

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Intern
Joined: 11 Jan 2015
Posts: 34
If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2016, 05:43
Official Explanation:

Quote:
4x(5n) is equivalent to 20xn, so to fi nd the value of t, you need
the value of xn. Statement (1) is sufficient: there is only one perfect square that
is less than 250 and divisible by 20: 100. (t must be divisible by 20 because x
and n must be integers.) Statement (2) is also sufficient: it directly gives us
the information we need: the value of xn. Choice (D) is correct.

For (1): While i understand that 100 is the only perfect square less than 250 and divisible by 20, i would like to ask you guys what´s wrong with the following approach:

Given from (1): 20xy < 250 -> xy < 12.5

So there are basically two perfect squares below 12.5 -> 4 and 9. So x and y (both integers) are either 2 and 2 or 3 and 3, hence the statement should be insufficient.

Can anyone (...or @Bunuel) explain the mistake in my thought process?

Thank you!
Senior Manager
Joined: 13 Oct 2016
Posts: 367
GPA: 3.98
Re: If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2016, 05:59
1
If 4x(5n) = t and x and n are integers, what is the value of t ?

(1) t is a perfect square less than 250.
(2) xn = 5

4 is already a perfect square, so our $$x$$ can be only of the form $$2^{even} = 2^0, 2^2, 2^4$$ ...

5 is not a perfect square, so in order for $$5n$$to be a perfect square $$n$$ should be of the form $$5^{odd} = 5, 5^3, 5^5$$...

(1) t is a perfect square less than 250

We can have following options:

(4*1)*(5*5) = 100 < 250

(4*4) * (5*5) = 400 > 250

(4*1)*(5*125) = 2500 > 250
...
All other options will be greater than 250. Sufficient.

(2) xn = 5

We have only one option which gives t as a pefect square - $$2^0*5^1$$. Hence x=1, n=5

4*25 = 100. Sufficient.

Senior Manager
Joined: 13 Oct 2016
Posts: 367
GPA: 3.98
If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2016, 06:38
1
Official Explanation:

Quote:
4x(5n) is equivalent to 20xn, so to fi nd the value of t, you need
the value of xn. Statement (1) is sufficient: there is only one perfect square that
is less than 250 and divisible by 20: 100. (t must be divisible by 20 because x
and n must be integers.) Statement (2) is also sufficient: it directly gives us
the information we need: the value of xn. Choice (D) is correct.

For (1): While i understand that 100 is the only perfect square less than 250 and divisible by 20, i would like to ask you guys what´s wrong with the following approach:

Given from (1): 20xy < 250 -> xy < 12.5

So there are basically two perfect squares below 12.5 -> 4 and 9. So x and y (both integers) are either 2 and 2 or 3 and 3, hence the statement should be insufficient.

Can anyone (...or @Bunuel) explain the mistake in my thought process?

Thank you!

I don't think that inequality is the right way to solve this question.

I'd better put it this way:

$$t^2 = 20xn$$

$$t = \sqrt{20xn} = 2*\sqrt{5xn}$$

For the expression under the radical to be an integer min value of $$xn$$ can be $$5$$. Either $$x=1$$ and $$n=5$$ or $$x=5$$ and $$n=1$$. In both cases we'll get our $$t$$.

If you wish you can put it in the form of inequality:

$$2*\sqrt{5xn} < \sqrt{250}$$

or

$$2*\sqrt{5xn} < 5* \sqrt{10}$$

But I don't think this will help much, because you'll need to approximate $$5* \sqrt{10}$$.

Regards
Manager
Joined: 17 May 2015
Posts: 249
Re: If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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20 Apr 2017, 21:22
If 4x(5n) = t and x and n are integers, what is the value of t ?

(1) t is a perfect square less than 250.
(2) xn = 5

Hi,

Given $$t = 20xn, t=?$$

(1) t is a perfect square less than 250.

$$t = 20xn = 2^{2}*5*x*n$$, In order to $$t$$ to be a perfect square either $$(x = 5, n = 1) \textrm{or} (n = 5, x = 1). => t = 100$$

Hence, Sufficient.

(2) xn = 5 => This statement is clearly sufficient.

Intern
Joined: 04 Sep 2013
Posts: 3
Re: If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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22 Apr 2017, 16:46
Given t = 20xn

Basic inferences
i. t is even
ii. units digit o t = 0

Statement 1
t is a perfect square less than 250.
100 is the only perfect square below 250 such that the units digit is 0
hence A is sufficient

Statement 2
XN = 5
we can calculate value of 20xn
hence B alone is sufficient

Manager
Joined: 01 Feb 2017
Posts: 175
Re: If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2017, 10:27
Ans B.

St1: t=20xn is a perfect square below 250 and x&n are integers (can be positive, negative or zero)
Therefore, there are two possible perfect squared values for t: 0 (x*n=0) and 100 (x*n=5)
Insufficient

St.2: xn=5; Therefore, t=100. Sufficient
Retired Moderator
Joined: 25 Feb 2013
Posts: 1220
Location: India
GPA: 3.82
If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2017, 11:00
Shobhit7 wrote:
Ans B.

St1: t=20xn is a perfect square below 250 and x&n are integers (can be positive, negative or zero)
Therefore, there are two possible perfect squared values for t: 0 (x*n=0) and 100 (x*n=5)
Insufficient

St.2: xn=5; Therefore, t=100. Sufficient

Hi Shobhit7

Well you have put forward an interesting perspective here. Some consider 0 to be a perfect square and some not.

A perfect square is always positive. If 0 is a perfect square, then it will be positive. but we know as per GMAT 0 is neither positive nor negative.

Hi Bunuel

The question seems to be ambiguous and the solution can have multiple interpretations. Can you clarify what is GMAT's take on 0 as a perfect square?
Math Expert
Joined: 02 Sep 2009
Posts: 52285
Re: If 4x(5n) = t and x and n are integers, what is the value of t ?  [#permalink]

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21 Dec 2017, 20:09
niks18 wrote:
Shobhit7 wrote:
Ans B.

St1: t=20xn is a perfect square below 250 and x&n are integers (can be positive, negative or zero)
Therefore, there are two possible perfect squared values for t: 0 (x*n=0) and 100 (x*n=5)
Insufficient

St.2: xn=5; Therefore, t=100. Sufficient

Hi Shobhit7

Well you have put forward an interesting perspective here. Some consider 0 to be a perfect square and some not.

A perfect square is always positive. If 0 is a perfect square, then it will be positive. but we know as per GMAT 0 is neither positive nor negative.

Hi Bunuel

The question seems to be ambiguous and the solution can have multiple interpretations. Can you clarify what is GMAT's take on 0 as a perfect square?

0 is a perfect square. The question is flawed.

--== Message from the GMAT Club Team ==--

THERE IS LIKELY A BETTER DISCUSSION OF THIS EXACT QUESTION.
This discussion does not meet community quality standards. It has been retired.

If you would like to discuss this question please re-post it in the respective forum. Thank you!

To review the GMAT Club's Forums Posting Guidelines, please follow these links: Quantitative | Verbal Please note - we may remove posts that do not follow our posting guidelines. Thank you.

_________________
Re: If 4x(5n) = t and x and n are integers, what is the value of t ? &nbs [#permalink] 21 Dec 2017, 20:09
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